This invention relates to a process for producing a titanium material having a layer with excellent corrosion resistance formed on the surface.
Titanium which itself has excellent corrosion resistance is being used in various field but has been used under increasingly severe corrosion environments in recent years, whereby there arise problems of general corrosion or crevice corrosion.
For solving such problems, there is the method of using corrosion resistant titanium alloys such as Ti-Pd, and there is also known the method of improving corrosion resistance by a surface treatment of titanium.
However, a corrosion resistant titanium alloy such as Ti-Pd has a drawback in that the cost becomes very high because an expensive noble metal is added. In the surface treatment methods, there have been developed the method in which palladium, ruthenium or oxide thereof is applied as a coating on the surface and the method in which titanium nitride or titanium carbide is bonded to the surface by ion plating or heat treatment in gases. However, in the former method, the cost becomes high because of the use of an expensive metal, while the latter method, which is specifically atmospheric annealing, requires troublesome steps and the heat treatment temperature exceeds the transformation point, whereby there is the problem of deterioration of the titanium material.
The present invention has been accomplished in view of the above situation, and as a result of various studies on the surface treatment methods for improving corrosion resistance of titanium, the present inventors have found a process for producing a titanium material which is very simple and has remarkably increased corrosion resistance.
Briefly, it has been found that the corrosion resistance of a titanium can be remarkably improved by permitting an oil to exist on the titanium surface at the time of cold working thereof, then causing the oil to adhere firmly onto the titanium surface by performing cold working and thereafter applying heat treatment at 300.degree. C. or higher temperature.
Based on this discovery, the present invention is intended to provide a process for producing very simply and inexpensively a titanium material of excellent corrosion resistance.